In recent years, development of laser is conspicuous. In particular, with respect to solid laser and semiconductor laser capable of radiating infrared rays having a wavelength of from 760 nm to 1,200 nm (may be referred to as “infrared laser”, hereinafter), those having a high output and small size have become readily available. Especially, in the field of lithographic printing, these infrared lasers are very useful as a recording light source in direct plate making of printing plates according to digital data of a computer, etc. Following this, various investigations have been also made about recording materials of a lithographic printing plate precursor sensitive to a variety of laser beams. There are known positive working recording materials sensitive to infrared laser having a photosensitive wavelength of 760 nm or more (see U.S. Pat. No. 4,708,925) and negative working recording materials of an acid catalyst crosslinking type (see JP-A-8-276558).
Usually, negative working image recording materials capable of undergoing recording upon exposure with infrared rays contain an infrared ray absorber that absorbs energy of infrared beam to convert it into heat, a radical initiator that generates a radical by the heat obtained from the infrared ray absorber, and a polymerizable compound that is polymerized by the generated radical as an initiator, and utilize a recording mode in which a recording layer of an exposed area (image recording material) is cured due to the occurrence and progress of polymerization reaction of the polymerizable compound to form an image. These negative working image recording materials involved such a problem that the sensitivity is low as compared with positive working image recording materials of causing solubilization of a recording layer by energy of infrared rays. Accordingly, image recording materials made of a polymerizable composition having high sensitivity to such an infrared recording light source, i.e., negative working image recording materials whose solubility in a developing solution largely lowers upon exposure with infrared rays are being demanded.
As image recording materials that are aimed to enhance the sensitivity, there are known those containing a component having excellent reactivity with light or heat such as chemical amplification type photosensitive materials (see JP-A-11-65105) and photosensitive materials utilizing polymerization of an ethylenically unsaturated compound (see JP-A-2000-89455). Further, with respect to the photosensitive materials utilizing polymerization of an ethylenically unsaturated compound, there are known photosensitive materials to which a reduction type additive is added (see JP-A-2002-82429), photosensitive materials to which a dithio compound or the like is added (see JP-A-2002-90985), and photosensitive materials to which a polycarboxylic acid compound is added (see WO 00/48836). However, any of these photosensitive materials have not yet been satisfied with practically useful sensitivity.
Also, these recording materials involved a fear of inferior storage stability such that an undesired reaction occurs in handling under white light or due to changes in the circumferential temperature, or polymerization hindrance is generated due to oxygen in air.